1. Field of the Invention
The present invention relates to the field of communication, and particularly to a baseband encoding/decoding method and apparatus for increasing the transmission rate over a communication medium while maintaining the desired bandwidth, bit error rate, and hamming distance.
2. Art Background
For several reasons, in the past few years there has been an increase in demand for data communication systems that can operate at high bit rates (such as 100 MBits/s). One reason for this increase has been the proliferation of data communication networks that can perform multi-media tasks, which require the communication of a large amount of data in a short amount of time. The increasing instruction execution speed of microprocessors also has resulted in the increased demand for data communication systems that can operate at high bit rates. Therefore, currently high bit rate data communication systems are being developed.
Implementations of these high speed data communication systems often confront numerous obstacles. For example, since the bit error rate (BER) increases proportionately with the bit rate and since the hamming distance decreases with the increasing BER, the maintenance of the desired BER and hamming distance are two constraints placed on the implementations of high speed data communication systems. Furthermore, solutions for maintaining the desired BER and hamming distance often have to comply with strict regulatory and design limitations placed on the bandwidth of the communicated data.
An example of one recently developed prior art data communication system, that maintains the desired BER, hamming distance, and bandwidth, while increasing the bit rate, is the 100BaseT4T+ Ethernet data communication system. This prior art system is designed to replace the 10 MBit Ethernet (10 BaseT) data communication systems, which utilizes a Manchester baseband coding method to send 10 MBits/s of data over two twisted pairs in a full-duplex mode (i.e., one twisted pair is used to receive data while the second twisted pair is used to transmit data). Like the 10 MBit Ethernet network, the 100BaseT4T+ Ethernet system is a baseband system, which renders it compatible with 10 MBit Ethernet networks.
On the other hand, unlike the 10 MBit Ethernet system, the 100BaseT4T+ Ethernet system uses four twisted pairs, three of which transmit data in a half-duplex mode (i.e., the three twisted pairs either all transmit or receive at any one time), while the fourth transmits control signals. In addition, because of the Federal Communication Commission's (FCC's) Part 15B requirement, which requires transmitted data signals to be 76 dBs below 0 dBm at 30 MHz, this 100 MBit Ethernet implementation does not use the Manchester pulse code modulation (PCM) scheme. If this 100 MBit Ethernet network used a Manchester PCM, a bandwidth of 50 MHz would be required, which in turn would violate the 30 MHz bandwidth restriction of the FCC. Consequently, in order to reduce the transmission bandwidth of this system below the 30 MHz FCC bandwidth requirement (i.e., reduce baud rate of this system to 25 MBaud), this 100 MBit Ethernet implementation uses an 8B-6T code (i.e., a code which represents eight binary bits-by six symbol pulses that can have any one of three values). In addition, the 8B-6T code enables the 100BaseT4T+ Ethernet system to increase the bit rate (from 10 MBits to 100 MBits) while maintaining the maximum BER and minimum hamming distance requirements set forth by IEEE 802.3 standard for 10 MBit Ethernet systems. More specifically, for 100 meters of unshielded twisted pair cable, the 100BaseT4T+ Ethernet maintains its BER at or below 10.sup.-9 and its hamming distance at or above 4, because it uses code redundancy techniques as it only needs 256 of the available 729 codes of the 8B-6T code.
Unfortunately, this prior art 100 MBit Ethernet network cannot readily be implemented in the United States because, in order to utilize the available present day Ethernet network infrastructure in the U.S., additional wiring has to be done. For example, the majority of the present day Ethernet networks use four twisted pairs to couple each data node (i.e., each data source, hub, and repeater). However, because hubs and repeaters need to receive bi-directional transmissions, four additional twisted pairs need to be placed between each hub and repeater and between each pair of repeaters, in order to implement the 100BasetT4T+ Ethernet system.
In addition, this prior art implementation cannot be easily adopted in the international arena because most Ethernet networks in foreign countries use only two twisted pairs to couple their communication nodes. Furthermore, because this prior art implementation uses four twisted pairs, it requires more electronic circuitry than an Ethernet network that uses two twisted pairs. In turn, the additional electronic circuitry makes this prior art 100 MBit Ethernet implementation more expensive than an implementation that uses less than four twisted pairs. Finally, by operating at only 25 MBaud, this 100 MBit Ethernet implementation does not optimally utilize the 30 MHz bandwidth that the FCC allows.